With a sudden rise of World Interoperability for Microwave Access (WiMAX), the 3rd Generation Mobile Communication System has to improve its network performance and reduce the costs of network construction and operation, to keep competitive in the field of mobile communication. Therefore, the standardization working group of 3rd Generation Partnership Project (3GPP) is now devoting to researching the evolution of Packet Switch Core (PS Core) and Universal Mobile Telecommunication System Radio Access Network (UTRAN). The subject of this research is called System Architecture Evolution (SAE).
The research aims to get the Evolved PS Core (E-PS Core) to provide higher transmission rate, shorter transmission delay, grouping optimization and to support mobility management of Evolved UTRAN E-UTRAN, UTRAN, Wireless Local Area Network (WLAN) and other Non-3GPP access network.
As shown in FIG. 1, presently, the architecture of SAE comprises the following network elements:
Evolved Radio Access Network (E-RAN), used to provide higher uplink /downlink rate, shorter transmission delay and more reliable wireless transmission. The network element contained in E-RAN is Evolved NodeB (eNodeB), which can provide wireless resource for user's access. The eNodeBs are interconnected with each other through the X2 interface.
Home Subscriber Server (HSS, used to store subscriber data permanently.
Packet Data Network (PDN is a network used to provide service for users.
E-Packet Core is used to provide shorter delay and more wireless access system access. E-Packet Core comprises the following network elements:
Mobility Management Entity MME is a control panel function entity used to store user data temporarily, responsible for managing and storing User Equipment (UE) context (e.g. UE/user identifier, mobility management state, user safety parameter), and distribute temporary identifier for users. When UE is located in a tracking area or a network, MME is responsible for user authentication, processing all the non-access layer message between MME and UE, and triggering paging in SAE. ENodeB and MME are connected through the S1-MME interface.
Serving Gateway (Serving GW) is a user plan entity, it is responsible for data routing process of the user plane, terminating downlink data of UE in idle mode, managing and storing SAE bearer context of UE (e.g. IP bearer service parameters and interior routing information of network). Serving GW is a user plane anchor in 3GPP system, and a user can be served by only one Serving GW at a time; eNodeB and Serving GW are connected through the S1-U interface. MME controls Serving GW through the S10 interface.
Packet Data Network Gateway (PDN GW) functions as gateway of UE access to PDN. PDN GW is a mobile anchor in 3GPP and non-3GPP access system. A user can access more than one PDN GW at a time.
Another notable feature of wireless mobile communication system is supporting mobility. In active state, the movement from one eNodeB to another without interrupting service is called a handoff process. In 3GPP, there are two kinds of handoff process between eNodes (1) handoff through the X2 interface, and (2) handoff through the S1 interface. The handoff through the X2 interface is illustrated as shown in FIG. 2, comprising the following steps:
S202, source eNodeB orders a UE to conduct a measurement, and after the measurement report is returned by the UE, the source eNodeB decides to switch to a target cell.
S204, the source eNodeB searches for a corresponding target eNodeB according to the target cell. If the target eNodeB can be found while connected with the source eNodeB via the X2 interface, the source eNodeB will issue a handoff request to the target eNodeB. The handoff request carries address information of the current MME and Serving GW and the user plane tunnel information of Serving GW.
S206, the target eNode determines whether there is a connection (or access) between the current MME and Serving GW. If there is a connection, the target eNode will conduct relevant resource reservation and then return a handoff response to the source eNodeB. Wherein, the handoff request carries relevant resource information.
S208, after receiving the handoff response, the source eNodeB will order UE to begin the handoff process, with resource information of the target eNodeB.
S210, after switched to the target eNodeB, UE sends a handoff acknowledgement to the target eNodeB.
S212, after receiving the handoff acknowledgement message, the target eNodeB sends a handoff completion message to MME to request MME to update relevant information. Wherein, the handoff completion message carries address information and the user plane tunnel information of the target eNodeB.
S214, MME transmits the address information and the user plane tunnel information of the target eNodeB to Serving GW. After the target eNodeB gets the user plane tunnel information and uplink/downlink data of Serving GW, transmission can be conducted between the eNodeB and Serving GW.
S216, Serving GW returns an updates bearer response to MME.
S218, MME returns a handoff completion response to the target eNodeB.
S220, the target eNodeB informs the source eNodeB to release resource.
The above handoff process is characterized in that the handoff between eNodeBs is not controlled by MME. The premises of the handoff above is that the two eNodeBs are connected through the X2 interface, and MME and Serving GW are unchangeable. This requires that one eNodeB is able to search out whether the other eNodeB has the X2 access and whether there is an access between a certain MME and Serving GW. It is necessary for the eNodeB to configure a certain MME, for eNodeB needs to find suitable MME at a user's primary registration. For Serving GW, however, it is MME that selects Serving GW in SAE, so there must be configuration of Serving GW in MME. If there was configuration of Serving GW in the eNodeB, the complexity of configuration of eNodeB would increase on one hand, and consistent configurations at the MME and eNodeB could hardly be assured on another hand.